The recovery of aluminum hydroxide from bauxite and similar alumina-bearing materials according to the Bayer process is achieved by digesting the ore with caustic liquor. The major portion of the alumina values are dissolved by the liquor, and the major portion of the unwanted ore constituents remain undissolved, making it separable from the liquor. The undissolved constituent is often referred to as "red mud". After pressure digestion of bauxite with caustic liquor, the red mud is removed from the sodium aluminate liquor by decantation and filtration, and the aluminum hydroxide is separated from the supersaturated sodium aluminate liquor, known as "green" or "pregnant" liquor, by precipitation. During the precipitation process, the supersaturated sodium aluminate liquor is cooled and mixed with a slurry of fine aluminum hydroxide which acts as seed to induce formation of its own species. Following the precipitation period, the slurry is pumped through a classification system. The coarse fraction of the crystallized aluminum hydroxide is separated from the sodium aluminate liquor and the resulting spent sodium aluminate liquor is recycled to be mixed with incoming bauxite in the digester.
High levels of impurities are undesirable in the "green" liquor used to produce aluminum hydroxide. Impurities decrease the whiteness or color purity of the aluminum hydroxide and, therefore, it is desirable to minimize those impurities in green sodium aluminate liquor before crystallization of aluminum hydroxide takes place. Typically, the impurities result in an aluminum hydroxide product having a whiteness of lower than 70% which greatly affects its use in commercial products where a high level of whiteness is required. The present invention solves the problem of poor whiteness and is capable of producing an aluminum oxide product having levels of whiteness powder of 90% or more. The whiteness level is relative to TiO.sub.2 which is considered to have a whiteness of 100%.
It is also well known that the presence of organic and inorganic impurities in a caustic sodium aluminate liquor causes process problems, lowers liquor productivity, and reduces the purity of the produced alumina. Difficulties caused by the organic impurities include lowered alumina yield, generation of excessive fine aluminum hydroxide particles, a higher impurity content in the alumina, colored liquor and aluminum hydroxide, lower red mud settling rate, loss of caustic due to formation of sodium organic compounds, increased liquor density, higher viscosity, raising of the boiling point, and foaming of the liquor.
Numerous methods are known for controlling and/or removing the organic material in Bayer process liquor. These include the treatment of the process liquor with sodium hypochlorite or other oxidizing agents such as oxygen or air. For example, German patent document Off. 2,945,152 describes a process for removing organic compounds from Bayer liquor by heating the liquor to 120.degree. to 350.degree. C. and introducing oxygen containing gas until a partial pressure of 3 to 30 atmospheres is reached. Inao et al U.S. Pat. No. 4,215,094 discloses a process for removing organic substances by contacting the aluminate solution with molecular oxygen containing gas in the presence of copper ions as a catalyst at an elevated temperature, e.g. 180.degree. to 300.degree. C.
It is also known to remove some impurities in a Bayer liquor by treatment with alkaline earth compounds. Schepers et al U.S. Pat. No. 4,046,855 teaches the treatment of aluminate liquor with a magnesium compound to remove organic materials. Mercier et al U.S. Pat. No. 4,101,629 treats a solution from the Bayer process with a barium compound to remove impurities. German patent document Off. 2,415,872 involves the addition of a calcium compound to the process liquor to remove the humic matter as insoluble calcium compounds.
Impurity levels of organic materials such as sodium oxalate have also been removed from sodium aluminate solutions as taught by Lever U.S. Pat. No. 4,275,042 by the use of cationic sequestrants comprising quaternary nitrogen compounds possessing medium and long chain alkyl groups and a single cationic charge. DeLaBretique U.S. Pat. No. 3,457,032 also discloses purification of a strongly alkaline solution such as sodium aluminate solution by treating the solutions with anion exchange resins of strongly basic and macroreticular type which are said to widely eliminate iron, silica, titanium, zinc, and organic acid impurities.
The removal of sodium oxalate from a sodium aluminate spent liquor solution by spraying the concentrated liquor onto a packing material is disclosed by Carruthers et al U.S. Pat. No. 4,038,039. Bush et al U.S. Pat. No. 4,496,524 teaches the removal of sodium oxalate from a sodium aluminate spent liquor by treatment with ethanol to cause the sodium oxalate to precipitate.
Yamada et al U.S. Pat. No. 4,280,987 removes carbon compounds from Bayer liquor by adjusting the molar ratio of the aluminum component to the sodium component and then heating the liquor to form sodium aluminate and drive off the carbon compounds as carbon dioxide.
Bird et al U.S. Pat. No. 4,282,191 describes the removal of zinc impurities from a caustic sodium aluminate solution using zinc sulfide seed to cause precipitation of the zinc in the liquor. Columbo et al U.S. Pat. No. 3,295,961 discloses a process for removal of iron impurities from the red mud slurry from a Bayer process by first drying the mud and then heating it to reduce the iron compound to metallic iron which is then separated from the dried mud using magnetic separation. Goheen U.S. Pat. No. 3,729,542 teaches the removal of iron impurities in a sodium aluminate solution by filtering the solution through a bed of iron particulate.
Conventional filtration is also, of course, known in the separation of sodium aluminate solutions from the red mud residue of a Bayer process digestion. For example, Corona U.S. Pat. No. 2,653,716 describes the cleaning of filter cloths used to separate sodium aluminate solutions from red mud.
The use of osmotic type filtration is known in the purification, for example, of water using reverse osmosis. Typical of such apparatus and associated processing are the disclosures in McLain U.S. Pat. No. 3,422,008; Coillet U.S. Pat. No. 4,161,446; Davis U.S. Pat. No. 4,367,132; and Klein et al U.S. Pat. No. 4,495,067. However, the purification of a highly caustic solution presents problems not normally encountered when purifying water. By highly caustic solution is meant solution from the Bayer process such as a sodium aluminate solution or caustic solutions having 1 wt. % or greater NaOH concentration or caustic solutions having a pH higher than 10.